/**
*      author: Y_Yao
*      created: 2025-05-06 21:48:21
**/

#include<bits/stdc++.h>

#define int LL
using namespace std;
using LL = long long;
const LL LLINF = 9e18;
const int INF = 0x3f3f3f3f;

void solve()
{
    string s;
    cin >> s;
    unordered_map<char,int> mp;
    for(auto& e : s)
    {
        mp[e]++;
    }

    for(char i = 'a';i <= 'z';i++)
    {
        if(!mp.count(i))
        {
            cout << i << endl;
            return;
        }
    }
}

signed main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);

    int t = 1;
    // cin >> t;
    while(t--)
    {
        solve();
    }
    return 0;
}